have a look at https://projecteuler.net/ https://projecteuler.net/ for "Thinker" brain food. (it still has the issue of not being a pragmatic use of time, but

have a look at https://projecteuler.net/ https://projecteuler.net/ for "Thinker" brain food. (it still has the issue of not being a pragmatic use of time, but there are plenty interesting enough questions which it at least helps)